High resolution multispectral mapping of the surface of the earth with air or spaceborne instruments requires optical objectives providing precise registration between counterpart detectors in the image planes of all spectral bands. Although there are numerous Schmidt optical systems for focusing multispectral images, those presently available systems which are sufficiently accurate to be used with multispectral instruments for mapping the surface of the earth must be optically linked to the mapping instrument by precision mechanical scanning mechanisms. The multispectral scanner (MSS) and the thematic mapper (TM) developed by the National Aeronautics and Space Administration are two examples of such instruments. Both use rapidly moving, mechanically driven, scanning mirrors to produce a whiskbroom scanning action of photodetectors upon ground resolution elements. Mechanical scanning mechanisms, however, inherently lose precision with continuous use, lack long-term reliability, generate undesired thrust forces which disturb other instruments, and have size limitations that limit the degree of resolution obtainable from a multispectral mapping instrument.
A French optical system known as a "Spot" instrument, currently under development, avoids the problems of mechanical scanning systems by using a two on-axis catadioptic Schmidt objectives boresighted to scan different, but adjoining fields of view. Orbital motion of the spacecraft sweeps a detector array across the earth in a direction perpendicular to the long dimension of the swath of earth scanned, producing a scanning effect known as pushbroom scanning. Eight linear detector arrays produce a four spectral band mapping system. The on-axis structure of the Spot instrument, however, produces a central observation which causes a degree of diffraction precluding operation of the system in a high resolution, short wavelength infrared (SWIR) mode. Also, the on-axis structure limits the space available for incorporation of a beamsplitter into the system which restricts application of the system to three, relatively closely spaced bands in the visible spectrum.